1. Field of the Invention
The present invention relates to an apparatus for detecting the standing wave ratio representing the degree of matching between a transmitter and an antenna or the like.
2. Description of the Prior Art
A circuit diagram of a generally-used conventional apparatus for detecting the standing wave ratio is shown in FIG. 1. In this drawing, reference numeral 1 shows a transmitter, numeral 2 a load (antenna), and numeral 3 a directional coupler. The coupler 3 includes a conductor section 31 for conducting an output signal of the transmitter 1 to the load 2, a traveling wave detecting conductor section 32 provided in parallel to the conductor section 31 for detecting the traveling wave F of the signal passing through the conductor section 31, and a reflected wave detecting conductor section 33 provided in parallel to the conductor section 31 for detecting a reflected wave R of the signal reflected from the load 2 to the transmitter 1 by the impedance mismatching between the transmitter 1 and the load 2. An end of each of the conductor sections 32 and 33 is grounded through a common resistor 34. Numeral 4 shows a traveling wave detector circuit connected to the other end of the traveling wave detecting conductor section 32 of the coupler 3 for detecting the traveling wave F detected by the conductor section 32, thus producing a traveling wave detected DC voltage V.sub.F. Numeral 5 shows a reflected wave detector circuit connected to the other end of the reflected wave detecting conductor section 33 of the coupler 3 for detecting the reflected wave F detected by the conductor section 32, thus producing a reflected wave detected DC voltage V.sub.R. Numeral 6 shows a change-over switch, the fixed terminal CAL of which is connected to the cathode of the detecting diode 41 of the traveling wave detector circuit 4. The fixed terminal SWR of the change-over switch 6, on the other hand, is connected to the cathode of the detecting diode 51 of the reflected wave detector circuit 5. The movable terminal of the switch 6 is grounded through a variable resistor 7 and a standing wave ratio indication meter 8. The cathode of the detecting diode 41 is grounded through a capacitor 42, and the anode thereof is connected to the traveling wave detecting terminal of the traveling wave detecting conductor section 32 of the coupler 3. The cathode of the detecting diode 51 is grounded through a capacitor 52, and the anode thereof is connected to the reflected wave detecting terminal of the reflected wave detecting conductor section 33 of the coupler 3.
In order to detect the standing wave ratio (SWR) in this circuit, first, the switch 6 is closed on CAL side so that the traveling wave detection DC voltage V.sub.F of the traveling wave detector circuit 4 is indicated on the meter 8. Next, the rheostat 7 is adjusted in such a manner that the pointer of the meter 8 is positioned at CAL (calibration) mark on the scale plate 81 of the meter shown by FIG. 2. The switch 6 is turned to the fixed terminal SWR, thus indicating the reflected wave detection DC voltage V.sub.R of the reflected wave detector circuit 5 on the meter 8. In this way, the ratio between the voltages V.sub.F and V.sub.R is detected. The indication on the meter 8 represents the value corresponding to the ratio between the traveling wave and the reflected wave. Therefore, if the standing wave ratio is calibrated on the meter 8, an indication on the scale plate 81 of the meter 8 shows a standing wave ratio.
In this type of conventional circuit, however, the change-over of the switch 6 and the adjustment of the rheostat 7, i.e., CAL adjustment are required each time of detection of the standing wave ratio, thus very much complicating the operation thereof.